April 28, 2005
Questioning the Signatures of Terrestrial Planets
Looking for biosignatures that would be characteristic of intelligent life is not always about extrapolating the most intelligent things a species might be doing. For instance, would one look for pollutants in the atmosphere? Carnegie Institutions' Maggie Turnbull answers that and other questions from colleagues following her lecture, "Remote Sensing of Life and Habitable Worlds".
Astrobiology Magazine -- Maggie Turnbull, an astronomer with the Carnegie Institution, has spent many years thinking about what kind of stars could harbor Earth-like planets. Her database of potentially habitable star systems could be used as a target list for NASA's upcoming Terrestrial Planet Finder (TPF) mission.
Turnbull presented a talk, "Remote Sensing of Life and Habitable Worlds: Habstars, Earthshine and TPF," at a NASA Forum for Astrobiology Research on March 14, 2005.
In this fourth and final part of the edited transcript, Turnbull answers questions from an audience of fellow scientists.
Question (Q): The Galileo spacecraft's main camera saw Earth as only a few pixels and tried to do spectroscopy on that. Will any such observations be done by the Cassini spacecraft?
Answer (A): I haven't heard anything about Cassini doing this, and now it is too late because, from Cassini's current point of view, the Earth is very close to the sun.
We are looking into getting spectra of the Earth with the Messenger mission, which is en route to Mercury but will be making an Earth flyby later in the year. There are some issues with the position of the sunshade, but that satellite has a low resolution spectrograph in the optical and the near infrared. That would be perfect if they could point it at the Earth, and, I think, the next logical step in preparation for TPF.
Q: Have you considered looking for biosignatures that would be characteristic of intelligent life, for instance, pollutants like CFCs that have a spectral signature in the infrared?
A: I've heard it mentioned in passing, but as far as I know that idea hasn't been pursued. I think it might be a problem of detectability. I don't know the wavelengths of all the stuff that we're spewing into the atmosphere. If they're dust particles or large molecules, they may be too far to the infrared for TPF to detect.
But methane is another biosignature that would be of interest, especially for younger planets. Methane on our planet today does to some extent reflect the presence of humans. But methane is hard to disentangle from geological activity, as we're now finding out with the detection of methane on Mars.
Q: Is there a maximum size for a terrestrial planet? How much does that detection envelope expand as you consider larger rocky planets?
A: No one really knows if you can make a rocky giant planet -- we have no analogue for this in our solar system. As far as we know, the maximum size of a terrestrial planet is somewhere between one and ten Earth masses, or between the mass of the Earth and the mass of Neptune.
As a planet gets larger, the fractional planet brightness goes up. So TPF will have a much easier time detecting larger terrestrial or giant gaseous planets, but we still want to mostly try to detect planets that are Earth-sized.
Q: How narrowly do we define something as being Earth-like? For large fractions of the Earth's history, the planet has been glaciated. The optical absorption in a forest is much greater than for the Earth as a whole, since oceans and glaciers have a different signature. So if you had a glaciated Earth or a heavily watered Earth, TPF could miss it completely.
You could still see the oxygen signature, but that brings up another issue: on Earth there are something like 20 different bacterial metabolic pathways. One happened to win the fight. Other photosynthetic pathways include Rhodopsin, a purple bacteria that has a different optical spectrum than chlorophyll. So what would a Rhodopsin World look like? That world would still be Earth-like, except for the fact that we couldn't live there.
A: Right. We should look into those different possibilities, and try to model what, spectrally speaking, an Earth would look like with those different and easily conceivable life forms dominating. But as far as biosignatures are concerned, unless you've got some sort of spectrally distinct organism covering the surface, I don't see how you can hope to detect it.
Glacier and water worlds would have very different spectra than the Earth, and they might still be perfectly habitable. Maybe looking for atmospheric signatures that are in strong disequilibrium with each other is one way to approach that problem.
Q: In your talk you said Mars would come into the habitable zone in about 2 billion years, but it looks like Mars was habitable maybe 3-and-a-half or 4 billion years ago. That suggests that the habitable zone could go out beyond 1.5 AU -- the sun was about 25 percent less bright 4 billion years ago, so Mars then would've been at the equivalent distance of about 1.8 AU. Maybe TPF should be looking for wider habitable zones. Planets further out are also easier to see from an angular separation standpoint.
A: Although, as far as detectability is concerned, at the outer edge of the habitable zone, the planets are getting fainter as you move them away from the star. So angularly the planets will be easy to see, but as far as fractional brightness goes, they will quickly become invisible.
Q: For binary stars, has anyone looked at the effects on the habitable zone of the luminosity of the more distant star?
A: You not only worry about that, but the radiation field is varying chaotically, because you've got the planet going around its main star and the second star also going around that main star. So the radiation field is constantly changing in a way that is not regular.
When I was choosing my Habstars, I threw out those that experienced a radiation field change of more than 3 percent. But that's pretty conservative -- I think 3 percent is not going to destroy all detectable life on a planet. I'm not quite sure where to draw that line, but I think we can draw it more generously than where I've drawn it, in which case it's the dynamical stability that's the big concern.
Q: Have you looked at going to shorter wavelengths to resolve planets closer to their host stars?
A: The big problem at shorter wavelengths is the Earth becomes pitch black, because the ozone layer absorbs everything. So the planet becomes invisible. Also, at shorter wavelengths you have fewer photons to work with, since sun-like stars emit less shorter wavelength light, and the planet itself is only reflecting that light.
Q: Will SIM and Kepler influence the selection of targets for TPF?
A: Kepler will give us some idea of how many stars we have to look at in order to have some hope of finding a terrestrial planet. If, in observing thousands of stars, Kepler finds that only one out of a hundred stars has a terrestrial planet, then we're going to be in trouble with a TPF core target list of only 35 stars, and we will have to rethink the mission.
The final target list for TPF will probably depend on what SIM finds as well. If SIM is not downscaled, and retains the full capability that is currently envisioned, then that mission will be able to put limits on the presence of planets more than a few Earth-masses located 1 or 2 AU from the star. This will be extremely helpful for choosing TPF targets.
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